Many broadband imagers are fielded in ground-based and airborne imaging systems. However, particularly for systems operating in the long-wave infrared (LWIR) spectral band, it can be difficult to obtain images with many pixels on a target, as may be needed to identify the target.
An alternative to broadband imaging is imaging spectroscopy. Spectral imagery may identify a target with as few as one pixel on the target. One type of interferometric spectrometer used to supply spectral data for many remote sensing applications is called a Fourier Transform Spectrometer (FTS). A common form of an FTS employs a Michelson interferometer with one arm having a variable optical path length. The variable optical path length may be implemented using a movable mirror. By scanning the movable mirror over some distance, an interference pattern or interferogram is produced that encodes the spectrum of the source. The FTS uses the Discrete Fourier Transform (DFT) or its faster algorithm, the Fast Fourier Transform (FFT), to convert the auto-correlation (each spectral amplitude encoded as the amplitude of a cosine signal) to physical spectra. The encoded spectrum is the Fourier transform of the source.
Referring to FIG. 1A, there is illustrated a block diagram of one example of an optical configuration of an FTS using a scanning Michelson interferometer implemented with a movable mirror. In this example, the FTS includes two mirrors 105, 110 with a beamsplitter 115 positioned between them. Mirror 105 is a fixed mirror and mirror 110 is a movable mirror. Electromagnetic radiation 120 incident on the beamsplitter 115 from a radiation source (not shown) is divided into two parts, each of which propagates down one of the two arms and is reflected off one of the mirrors. Radiation 120a in a first optical path is reflected by the beamsplitter 115 and reflected by the fixed mirror 105. On the return, the radiation 120a is again split by the beamsplitter 115, such that 50% of the radiation is reflected back to the input, and the remainder is transmitter through the beamsplitter to a focal plane array 125. Radiation 120b in a second optical path is transmitted through the beamsplitter 115, and reflected by the movable mirror 110 which imparts a modulation to the radiation (motion of the mirror 110 is indicated by arrow 130). On the return, the radiation 120b is also again split by the beamsplitter 115 such that 50% of the radiation is transmitted through the beamsplitter back to the input, and the remainder is reflected to the focal plane array 125. The two beams are recombined at the focal plane array 125. When the position of the movable mirror 110 is varied along the axis of the corresponding arm (indicated by arrow 130), an interference pattern, or interferogram, is swept out at the focal plane array 125 as the two phase-shifted beams interfere with each other.
FIG. 1B illustrates an alternative configuration of an FTS. In this configuration, two focal plane arrays 125a, 125b are used, and the fixed mirror 105 and moving mirror 110 are oriented such that approximately 50% of the radiation 120a, 120b from each optical path is directed to each focal plane array. The spectra from each focal plane array 125a, 125b may be averaged to improve the overall signal-to-noise ratio. This configuration avoids the radiation loss associated with the configuration of FIG. 1A, but is more complex and requires additional components.